1. Field of the Invention
The present invention relates to a power supply device and an image forming apparatus.
2. Description of the Related Art
As an auxiliary power supply, a secondary battery and a capacitor are widely used. Generally, as a main power supply, for example, a solar cell, an aerogenerator, or a fuel cell is used. Alternatively, a commercial alternating-current (AC) power supply or an engine generator can be used as the main power supply. For example, in an electrophotographic printer, to expedite heating of a fixing unit for thermally fixing a toner image on a sheet, not only a main power supply but also an auxiliary power supply may be necessary to supply a sufficient power to the fixing unit.
Generally, a charging circuit (charging unit) is connected between the main power supply and the auxiliary power supply. In addition, to prevent overcharging, a charging control circuit is used to stop charging the auxiliary power supply when the power charged in the auxiliary power supply is saturated.
Furthermore, a diode for preventing a back flow is inserted in a charging line (PFf and PFb) between the charging circuit and the auxiliary power supply to prevent a discharge of the auxiliary power supply toward the charging circuit when the charging unit stops charging the auxiliary power supply. While the auxiliary power supply is charged, a current flows forward in the diode. However, the diode has a forward voltage drop that causes a power loss. Specifically, when a typical pn junction diode is used, which is the easiest method, the pn junction diode has a forward voltage drop of about 0.7 volt. Alternatively, a Schottky diode having a forward voltage drop of 0.3 volt to 0.4 volt can be used. However, even when the Schottky diode is used, the power loss due to its forward voltage drop cannot be ignored in some power supplies, such as a solar power generator, a wind power generator, and a high-efficient auxiliary power supply. To prevent such a power loss, a method using a diode and a switching unit, such as a relay switch or a semiconductor switch, connected in parallel can be adopted. In this method, the switching unit is turned on (closed) during charging, and the switching unit is turned off (open) while the charging stops to prevent the back flow from the auxiliary power supply to the charging circuit.
For example, Japanese Patent Application Laid-open No. H2-168819 discloses a technology using a back-flow preventing unit with a power loss lower than that of the Schottky diode. The back-flow preventing unit includes a diode and a switch connected in parallel. The switch includes a transistor having a forward voltage drop lower than that of the diode. In the instance when a main power supply is cut off, the switch is turned off, thereby preventing a back flow.
However, in the technology disclosed in Japanese Patent Application Laid-open No. H2-168819, the switch is turned off only when the main power supply of the charging circuit is cut off and the technology cannot be used to prevent a back flow caused when the voltage of the auxiliary power supply on a side of a load is higher than an output voltage of the charging circuit during the charging, which is problematic.